Solve a common upgrade issue with the built in web server.
This only applies if you are using the built in web server, not IIS.
An elevated command prompt.
In the command prompt, run
netsh http add urlacl url=http://*:<<port>>/ user=<<user>>
where <<port>> is the port you have set the SQL Monitor web service to use, and <<user>> is the user the web service is running as.
You will then need to restart the SQL Monitor BaseMonitor Service, and the SQL Monitor Web Service.